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in the case of saving data a custom table? should I also call up the table?

example my table where i save the custom fields is called custom_mb_field

i tried following this guide
/**
* Action: 'rank_math/vars/register_extra_replacements' - Allows you to add extra variables.
* Snippet to register the variable that will return the number of posts in the current term.
*/
add_action('rank_math/vars/register_extra_replacements', function () {
rank_math_register_var_replacement(
'term_count',
[
'name' => esc_html__('Term Count', 'rank-math'),
'description' => esc_html__('Number of posts in the current term', 'rank-math'),
'variable' => 'term_count',
'example' => term_count_callback(),
],
'term_count_callback'
);
});
term_count_callback() function
{
$term = get_queried_object();
return isset($term) ? $term->count : null;
}

available at this link https://rankmath.com/kb/variables-in-seo-title-description/#how-to-add-custom-variables

and I tried saving it inside wpcodebox2 but it always returns the empty field